The present invention relates to a content playback method and a recording and playback method that use content storage media with a copyright protection function, especially removable content storage media
Digital broadcasting in recent years encrypts content data by adding a limiting signal, such as “copy once” signal, to it from the standpoint of copyright protection. Compatible recorders therefore encrypt content data on their own and store it on a hard disk drive to prevent an illicit use of or inadvertent leak of the content data due to human errors. When content data recorded in a hard disk drive is written into a DVD or Blue-ray Disk, the content data stored in the hard disk drive is deleted (by a move function), thus conforming to a rule of “copy once”.
However, there is a problem with this method. That is, since a recorder encrypts content data on its own, a removable hard disk drive cannot be used, which is inconvenient to the user. Under this circumstance, a removable hard disk drive “Secure iVDR” with a copyright protection function has become available on the market. The Secure iVDR has a function of independently managing usage passes, which include encrypted content data and its decryption key, a function of protecting stored usage passes against illicit access, and a function of transferring usage passes with a PKI-based bidirectional authentication, and realizes a copyright protection of content data which have been separately stored from the usage passes.
Further, for music players to play music content and for video players to play video content, specifications have been defined using this content copyright protection function. In the case of a video player, a logical block of contiguous 3072 bytes obtained by dividing a video stream is called an access unit (AU) and encryption is performed in units of AU. A logical block of contiguous 512 AUs is called an allocation unit (ALU), which constitutes a minimum unit in which video data is encrypted by one usage pass. In realizing a move function for content data, the Secure iVDR is required to change the usage pass in less than one minute, considering the fact that the time during which content data are allowed to overlap each other is less than 1 minute (see Recording and Playback Device for iVDR—TV recording specification (http://www.safia-1b.com/).
In a recorder using such a Secure iVDR, a normal playback is realized by performing a mutual authentication between a recorder body and the Secure iVDR, establishing an encrypted communication path using a usage pass transfer protocol between the recorder body and the Secure iVDR, safely reading the usage pass, decrypting separately read encrypted content data with an encryption key saved in the usage pass, and transferring the decrypted data to a decoder. However, since the recorder body and Secure iVDR use a built-in CPU, there is a possibility that safely writing/reading the usage pass may take much time.
On the other hand, a hard disk recorder in general can simultaneously play a content that is being recorded in a mode called a follow-up playback. This type of viewing is done when the user temporarily leaves a television he is watching. On leaving the TV, the user starts this mode of recording and, when he returns, resumes viewing the video from where he left off. In this type of viewing, it is considered necessary to get the playback state as close to the currently broadcast state as possible (or to catch up with the current broadcast state).
In the Secure iVDR or removable hard disk drive, a recording needs writing the usage pass into a hard disk drive and a playback requires reading the usage pass from the hard disk drive.
For example, in JP-A-2005-285287, since the follow-up playback is performed by using a content key held in a content key generation module without reading the content key from the storage medium hard disk, the key information is held in memory.
The follow-up playback is a playback that reads content data recorded in a hard disk drive.
Under these premises, to realize a catch-up, it is necessary for the playback processing side to be able to successively locate a current recording position (hard disk writing position). In this case, it is possible to get close even to the unit data block being written into the hard disk.
Further, when the Secure iVDR is used, the playback processing side must also be able to safely use the content data key information usage pass. For this purpose, one method may involve, as in normal processing, writing a usage pass into a hard disk drive and reading it. However, since this operation is executed by a CPU built into the Secure iVDR, the writing/retrieval of the usage pass takes time. If, for example, it takes a few seconds, the playback that can be realized can only play the content data lagging the currently broadcast state with a delay of a few seconds. So, the catch-up cannot be realized.
In JP-A-2005-285287, since the playback is performed by using a content key held in the content key generation module, an overhead of reading the content key from the hard disk drive can be eliminated. However, JP-A-2005-285287 provides one encryption key for the content data and does not contemplate a situation where the content key may change in a playback time of as short as one minute.
If the content key is to be saved always in memory, it is necessary, in the case of 120-minute content data for example, to have at least 120 pieces of key information. Each piece of key information needs at least 64 B, so about 8 kB of memory is required. If information on this memory should leak, a decryption key for entire content data can become available for a third person, which poses a security problem.
Further, if an arrangement is made to hold only one encryption key in memory and, when recording is finished, store it in the hard disk drive, the catch-up time cannot be made less than one second at the timing of writing the encryption key because the writing takes time (of the order of seconds).